1. Field of the Invention
This invention relates to automatic transmission oil compositions.
2. Prior Art
An automatic transmission in an automobile is a mechanism designed to make automatic setting of torque ratios according to the speed of travel and the amount of load. This mechanism comprises a torque converter, a multiplate clutch/planetary gear and an oil pressure regulator that are all lubricated by a common transmission oil. The oil pressure regulator detects delicate changes in the car speed and load, thereby controlling the overall operation of the transmission. The oil in the torque converter and clutch/gear sections is subjected to severe shear which tends to break off the molecules of the high molecular viscosity index improver that is present in the oil, resulting in reduced oil viscosity. This must be suppressed to prevent lower lubrication and reduced oil pressure leading to unstable operation of the transmission system. Subjecting the transmission oil to severer shear is anticipated by the introduction of a continuously variable transmission (CVT) which outdates the conventional automatic transmissions. A keen demand is therefore called for an improved lubricating oil which has the requirements of all types of automative transmission and which is in particular capable of holding a viscosity loss below 10% with respect to fresh oil under varying operating conditions.
Ordinarily, transmission oils are used commonly throughout all seasons from cold to hot environment and therefore should desirably be least susceptible to changes in viscosity with temperature and less viscous at lower temperature. Too low viscosity with elevated temperature would fail to build sufficient oil pressure, and conversely too high viscosity with low temperature would lose oil fluidity. With this in view, the transmission oil should normally have a viscosity of above 7 cSt at 100.degree. C. and below 50,000 cp at -40.degree. C.
Automatic transmission oil compositions in conventional use typically comprise mineral oils or synthetic oils blended with a viscosity index improver such as methacrylic acid ester copolymers and styrene/ester copolymers. While these transmission oils are satisfactory in viscosity-temperature characteristics and low temperature fluidity, they are not totally satisfactory with respect to viscosity against mechanical shear. This problem could be coped with, as appears obvious to one skilled in the art, by reducing the average molecular weight of the aforesaid viscosity index improvers. However, such approach is impractical where higher shear stability is required.